fix: list only oncall users in plugin

[t1agob]

Description

Current implementation lists all oncall users, on all escalation levels, for an escalation policy assigned to a specific service. This is how the PagerDuty API works currently.

Problems:

• If an escalation policy contains an oncall schedule, then the user currently on-call is returned. But if individual users are used instead all of them are listed.
• Same user might exist in different escalation levels which causes the same user to show up multiple times on the list.
• The current implementation doesn't use the gravatar image used in PagerDuty. It uses a dummy avatar icon for everyone.

Proposed implementation:

• List only users in escalation level 1 - these are the users that are actually oncall.
• Remove duplicate users from the oncall user list
• Add support for user profile picture and fallback to dummy avatar icon when an image is not provided.

Issue number: #36

Type of change

• New feature (non-breaking change which adds functionality)
• Fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• Changes generate no new warnings
• PR title follows conventional commit semantics

If this is a breaking change 👇

• I have documented the migration process
• I have implemented necessary warnings (if it can live side by side)

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